When optical parameters of liquids are measured from separate samples, a drawback in measuring samples taken from process liquids of industrial processes, for example, is that foreign substances in the samples, or organic components often found in the mixture, tend to accumulate on the optical surfaces of optical windows, such as measuring prisms. This can take place, for example, when a sample solution is in a static, non-flowing state. As an example of measurements concerning optical parameters of solutions, measurements taken with a refractometer, for example, are described in the U.S. Pat. No. 6,967,151.
Sedimentation relating to measurements of solutions including solid matters can be an issue. A solid matter can find its way, as a result of gravity, towards the bottom of a measurement vessel, such as a sample vessel.
Concerning oil-based emulsions, for example, is the separation of oil into a different phase. This may also take place when measuring milk samples, for example, in which fat globules find their way to the surface of the liquid both on the walls of a sample vessel and the optical surfaces of the measurement window.
Due to surface potential characteristics, certain substances can possess the trend that, over time, their concentration increases right next to the surface. When sugar solutions or solutions of the wood-processing industry, for example, are measured, the measurement result has the tendency of increasing as a function of time.
Attempts to solve the aforementioned issues have been made by using mixers of various kinds in connection with a sample vessel. As mixers, either small, blade-type, whisk-type or magnet pill type mixers have been used, arrangements in which a magnet element is placed in the measurement vessel, which can be made to rotate by means of a magnetic field and to mix the liquid. Mechanical rotation or back-and-forth movement may be incorporated into the above arrangements.
For small capacities, typically approximately 5-10 ml, an adequate local rate of flow cannot be focused on optical measurement surfaces. If a mixer's rotation speed is increased, the result with whisk-type mixers, for example, is the mixing of air into the sample, which in turn leads to foaming. A mixer in a small sample cuvette cannot be installed eccentrically enough in relation to the measurement cuvette. The result is that even a very low rotation speed can create a vortex in the cuvette, and air can be mixed into the sample, or the sample can escape from the center of the cuvette to the edges. The optical measurement surface can be located at the bottom of the cuvette, so in a situation like this it is easily left without a sample.
Due to the issues described in the above, the known solutions do not allow a strong enough local flow rate to be achieved towards the optical surfaces of a sample vessel so as to remove the build-up effects the sample creates, or to maintain efficient mixing of the sample.
Many of the known measurement devices are also provided with thermostats to keep the temperature of the sample being measured constant. The previously known mixing arrangements do not shift the vertical layers of the sample efficiently enough, whereby the measurement result is not the best possible.